+1 to vinegar
I think this could be a function of how it is stored (and possibly shipped).
I've only had Grand Cru in Belgium (its tough to come by in the Midwest). IMO it was far less acidic than Gueuze, and probably less so than most Flanders Reds (Duchesse, for example).
(Relatively) fresh, the beer didn't have an overwhelming acetic bite. It is there, but pleasing (at least to me), just sharp enough to overcome the residual sweetness.
Acetic acid may just be something that is all-together off-putting to some palates. I have a colleague that despises any foods with the slightest trace of vinegar: pickles, salad dressing, etc.
To me, some amount of acetic acid in Flanders Red/Browns can add depth, complexity, and the right kind of acidity. A touch is definitely traditional and appropriate in terms of "style", but it tends to offend judges. Its a tricky balance that may be thrown off by age and poor storage conditions.